fbcode_builder: getdeps: add list-deps subcommand

Summary:
While the command isn't necessarily super useful
on its own, it does show that the plumbing for walking the
deps is functioning, and that is important when it comes
to building.

The output lists the projects in the order that they
would be built.

The `fetch` command has been augmented to add a `--recursive`
flag that uses the same mechanism to recursively fetch
the dependencies.

Reviewed By: simpkins

Differential Revision: D14691004

fbshipit-source-id: b00bf6ad4742f8bb0a70698f71a5fe03d6a1f453

build/fbcode_builder/getdeps/platform.py

@@ -96,3 +96,21 @@ class HostType(object):
             and self.distro == b.distro
             and self.distrovers == b.distrovers
         )
+
+
+def context_from_host_tuple(host_tuple=None):
+    """ Given an optional host tuple, construct a context appropriate
+    for passing to the boolean expression evaluator so that conditional
+    sections in manifests can be resolved. """
+    if host_tuple is None:
+        host_type = HostType()
+    elif isinstance(host_tuple, HostType):
+        host_type = host_tuple
+    else:
+        host_type = HostType.from_tuple_string(host_tuple)
+
+    return {
+        "os": host_type.ostype,
+        "distro": host_type.distro,
+        "distro_vers": host_type.distrovers,
+    }